The present invention relates generally to charging circuits for charging batteries. More particularly, the invention relates to an efficient battery charger for charging batteries that are sensitive to high temperatures during charging.
Over the past couple of decades the use of portable electronic devices has increased dramatically. Most portable devices are powered by either primary batteries such as alkalines, or by rechargeable batteries such as NiCad and lead-acid. Primary batteries are capable of powering portable devices for long periods of time, but are useless after the charge stored in them has been depleted. Rechargeable batteries, on the other hand, can be reused multiple times after being charged, but they are only capable of powering devices for relatively short periods of time. This has stimulated research into the development of rechargeable batteries that are capable of powering devices for longer periods of time. The research has led to the development of Nickel Metal Hydride (NiMH) batteries. NiMH batteries are capable of storing twice as much energy as NiCad batteries, which results in a duration of use almost twice as long as the NiCads. However, a major drawback of NiMH batteries is that during charging they are less tolerant to heat than NiCads and other rechargeable batteries. Charging a NiMH battery at elevated temperatures can potentially lead to destruction of the device, possibly resulting in damage to the battery charger. The sources of heat the battery is subjected to include the ambient temperature, the heat radiated from the battery charger during quiescent operation, the heat radiated from the battery charger while charging a battery, and the heat radiated by the battery during charging. The sensitivity of NiMH batteries to high temperatures demands that the battery charger design minimize the heat radiated during all operating modes.
Conventional battery chargers have been designed for use with NiCad and alkaline batteries. Since these types of batteries are not nearly as sensitive to heat during charging as NiMH batteries, there has not been the stimulus to create chargers that minimize radiated heat. Therefore, battery chargers that are designed for NiCad and alkaline batteries dissipate a significant amount of power resulting in relatively high assembly temperatures during battery charging. A significant portion of the dissipated power is generated in a portion of the circuit referred to as the bootstrap network. The amount of power dissipated in the bootstrap network is directly related to the level of the input voltage to the circuit. As the input voltage increases, the power dissipated in the bootstrap network increases. As a result of the power losses in bootstrap networks and the sensitivity of NiMH batteries to high temperatures, conventional chargers that are powered from a 115 Vac input source provide marginal operation when charging NiMH batteries. Moreover, charging NiMH batteries from conventional chargers that are powered from a 230 Vac input source, with their attendent high assembly power dissipation, can result in the destruction of the batteries being charged. The high temperatures associated with conventional chargers powered from 230 Vac thus makes them unsuitable for charging NiMH batteries.
There are two main circuit configurations that are employed for the internal design of battery chargers: constant current chargers and voltage switching chargers. Constant current chargers regulate the battery charging current by varying the impedance of a series pass device, essentially dissipating power in the pass device to maintain regulation. Due to the relatively high assembly power dissipation, constant current chargers are unsuitable for charging NiMH batteries and typically are used only for charging low capacity batteries. Voltage switching chargers generally have a switch in series with the input voltage source, an inductor, and the battery that is being charged. The switch is repetitively cycled, thereby applying the input voltage to the inductor which provides an averaged current to the battery. Both, constant current chargers and voltage switched chargers require a circuit to provide power from the input power source to drive the series pass device or the series switch. A majority of the losses in battery charger circuits occur in this circuit which is referred to as the bootstrap network. As the input voltage to the charger is increased the losses in the bootstrap network increase substantially. To minimize the power dissipated, the resistance of the network must be maximized. However, the resistance of the bootstrap network must be maintained low enough to provide sufficient power to operate the switching device. Therefore, there is a maximum limit on the resistance of the bootstrap network beyond which the battery charger circuit will not operate. Conventional battery chargers that have been designed with the maximum operable bootstrap network resistance continue to generate more heat than is acceptable for charging NiMH batteries. When operated from a 115 Vac input, or 155 Vdc input, conventional circuits provide marginal charging performance. When the input source is 230 Vac, or 300 Vdc, the heat generated from the bootstrap network losses can make conventional battery chargers unsuitable for charging NiMH batteries.
The present invention solves the excessive heat problem of conventional battery chargers by using a novel low dissipation bootstrap network. Whether powered from 155 Vdc or 300 Vdc, the invention significantly reduces the losses in the bootstrap network. The reduced power losses in the bootstrap network result in lower temperatures to which batteries under charge are exposed. The lower ambient temperatures permit reliable, optimum charging of NiMH batteries.
The present invention minimizes power dissipation in the bootstrap network by reducing reliance on the bootstrap network as the charge path for the local energy storage capacitor that provides power to the series switch. The input power is passed through a switching device which intermittently applies the input voltage to an output inductor. An auxiliary winding on the output inductor provides an isolated source of energy that is coupled back into the local energy storage capacitor. Coupling energy from the auxiliary winding reduces the amount of charge that must be provided through the bootstrap network, thereby permitting a larger bootstrap resistance to be selected. The larger bootstrap resistance minimizes losses in the network, reducing the overall battery charger heat generation, resulting in a lower battery charger operating temperature. Thus, the low power dissipation battery charger provides a low temperature environment for charging batteries.
In view of the above, it is a primary object of the present invention to provide a battery charger circuit that is capable of providing a reliable, inexpensive, low temperature environment for charging NiMH batteries.
It is a further object of the present invention to provide an inexpensive battery charger circuit that can operate from 300 Vdc, that is capable of providing the low temperature environment that is appropriate for recharging NiMH batteries.
It is yet another object of the present invention to provide a battery charger circuit in which the power dissipation of the assembly is minimized, thereby providing an energy efficient circuit for charging a variety of rechargeable batteries.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.